1. Field of the Invention
This invention relates to a bonded wafer and a method for producing a bonded wafer, and more particularly it is to suppress residual oxide in a bonded face as far as possible.
2. Description of the Related Art
As a typical production method of a bonded wafer, there are known a method wherein two silicon wafers are bonded through an insulating film such as an oxide film or the like or directly bonded without the insulating film and then a wafer for active layer is grinded and polished to form an active layer (grind-polishing method), and a method wherein ions of hydrogen or the like are implanted into a surface layer portion of a silicon wafer for active layer to form an ion-implanted layer and bonded to a silicon wafer for support substrate and then subjected to a heat treatment to exfoliate the wafer at the ion-implanted layer to thereby form an active layer (ion implanting separation method) or so-called smart cut method (registered trade mark) (see JP-A-H11-67701 and JP-A-H05-211128).
However, when the two silicon wafers are directly bonded after the removal of a native oxide by immersing in a HF solution, or particularly when two silicon wafers having different crystal orientations (for example, (110) face and (100) face) are directly bonded without using an insulating film such as an oxide film or the like, an oxide trapped in an interface therebetween may locally concentrate and retain in the interface as a step-like oxide in the production step of the bonded wafer (heat treatment step). As a result, there is a problem that such a residual oxide deteriorates the device characteristics but also forms a nucleus of defect in the preparation step of the device.